As a process for the refining treatment of crude oil, there has heretofore been adopted the process which comprises the steps of atmospherically distillating crude oil to separate each of the fractions and thereafter desulfurizing the respective fractions thus separated. The above-mentioned process, however, is not necessarily satisfactory, since it involves the problems that a large number of refining apparatuses are required, the steps are intricate and troublesome, repeated cooling and heating of the petroleum products lower the energy efficiency and the like. In view of the above, a novel system of petroleum refining is eagerly desired.
From such viewpoint, an attempt has recently been made to collectively treat crude oil from which naphtha fraction is removed. There have been proposed, for example, (1) a process comprising the steps of distilling away naphtha fraction in crude oil; then collectively hydrodesulfurizing the residual petroleum free from naphtha fraction; and subsequently distilling the hydrodesulfurized petroleum to separate it into respective petroleum products (refer to Japanese Patent Application Laid-Open No. 294,390/1991), (2) a process comprising the steps of distilling away naphtha fraction in crude oil; then collectively hydrodesulfurizing the residual petroleum free from naphtha fraction; subsequently separating the hydrodesulfurized petroleum in a high pressure separating vessel into light fraction and heavy fraction; and then hydrorefining the light fraction thus obtained (refer to Japanese Patent Application Laid-Open No. 224,890/1992), (3) a process comprising the steps of distilling away naphtha fraction in crude oil; then collectively hydrodesulfurizing the residual petroleum free from naphtha fraction; subsequently separating the hydrodesulfurized petroleum in a high pressure separating vessel into light fraction and heavy fraction; catalytically cracking the heavy fraction thus obtained at around 500.degree. C. in nitrogen atmosphere under around atmospheric pressure to obtain gasoline and light cracked oil (LCO); and then hydrorefining the LCO thus obtained and the light fraction which has been separated under high pressure (refer to Japanese Patent Application Laid-Open No. 224,892/1992), (4) a process comprising the steps of subjecting crude oil to collective treatment and atmospheric distillation; then subjecting the residual oil thus obtained to fluidized catalytic cracking or hydrocracking; and adjusting the product yields (refer to U.S. Pat. No. 3,617,501), (5) a process in which proper operating conditions having high economical efficiency are provided in a collective hydrotreatment of crude oil highly liable to metallic contamination by means of a moving-bed type reactor capable of continuously replacing a catalyst, in particular, a process in which continuous operation period of the production process is prolonged as compared with conventional processes and at the same time, the content of nitrogen, a metal or asphaltene in the residual oil is decreased by the combination of contaminant removal using the first-stage countercurrent moving-bed type reactor and hydroreforming using the second-stage fixed-bed type reactor, and the like processes.
However, with regard to the above-mentioned process (1), the use of a conventional desulfurization catalyst makes it impossible to produce kerosene and gas oil fractions that are stabilized in quality, and besides to exhibit satisfactory effect on the increased production of clear oils. Specifically, in the case where a conventional fixed-bed type reactor is used in the process (1), the continuous operation period of the production process is not satisfactory, and besides the product properties of each fraction, for example, nitrogen contents and color tones of kerosene and gas oil, the smoke point of kerosene, or the content of nitrogen, a metal or asphalten in the residual oil have been inferior to the properties of the products obtained by a conventional refining process. With respect to the aforestated process (2), although the properties of the kerosene and gas oil are improved, the smoke point of the kerosene is not fully satisfactory depending on the purpose of use, the usable crude oil is limited depending upon the demand constitution, and besides there is caused such a problem that the treating equipment is complicated because of the practice of desulfurizing treatment, followed by the hydrorefining, thereby inevitably increasing the equipment cost and running cost. As to the above-mentioned process (3), the production of gasoline is increased along with the production of LCO having the boiling point range corresponding to that of kerosene and gas oil, but the LCO thus obtained is extremely high in aromaticity and is remarkably low in the smoke point of the kerosene fraction and in the cetane value of the gas oil fraction. In order to achieve satisfactory smoke point or cetane value by hydrogenating such LCO, it is necessary to provide a high temperature and pressure unit withstanding a high degree of severeness and also to reboost the LCO up to the reaction pressure. Accordingly there has not yet been attained economical efficiency satisfactory in both fixed cost and variable cost. In regard to the process (4), the intermediate fraction obtained by fluidized catalytic cracking is extremely poor in its quality, including for example, the hue of gas oil or kerosene, the smoke point of kerosene, the cetane index of gas oil, etc. On the other hand, it is imperative in the hydrocracking, that the temperature and pressure which have once been lowered in the atmospheric distillation should be raised again to a high temperature and pressure, that is, to 300 to 450.degree. C. and 100 to 200 kg/cm.sup.2, and therefore, this process itself is not necessarily satisfactory from the viewpoints of energy efficiency and economical efficiency. Regarding the above-described process (5), although the residual oil is improved in its quality by carrying out collective desulfurizing treatment of crude oil or crude oil from which naphtha fraction is removed, by the use of a conventional desulfurization catalyst, there is caused such a problem that the kerosene and gas oil components are unsatisfactory in the quality, for example, smoke point and hue stability. Thus it has been clarified that the single use of the moving-bed type reactor is devoid of practicability for the collective treatment. Moreover, since the moving-bed type reactor must be applied to crude oil highly contaminated with metals (for example, not less than 150 ppm), that is, a heavy fraction from the economical point of view, there is a problem that the usable crude oil is restricted.
The actual circumstance at the present time of the conventional collective treatment process for crude oil from which naphtha fraction is removed is that the above process is not yet brought to practical application owing to the difficulty in producing kerosene and gas oil fractions stabilized in qualities, insufficiency in continuous operation period of the production process and expensiveness in equipment cost and running cost.
On the other hand, the hydrogenating treatment of heavy hydrocarbon oil containing asphaltene (heptane-insoluble matter), sulfur, metallic components and the like has involved the problem that the catalyst markedly deteriorates by the accumulation of metallic components, carbonaceous deposits on the catalyst during the course of the treatment. As a process for prolonging the service life of the catalyst, there are proposed, for example, (6) a process comprising the steps of passing crude oil through one of preliminary reactors that are installed in parallel on the upstream side of a principal reactor and, after the deterioration of the catalyst, passing crude oil through the other preliminary reactor to maintain the catalyst activity (refer to Japanese Patent Publication No. 6163/1974); (7) a process comprising the steps of passing crude oil through a reactor which is divided into the former stage packed inside with a demetallization catalyst and the latter stage packed inside with a desulfurization catalyst, and, after the deterioration of the catalytic performance, withdrawing the catalyst in the former stage to replace it with a fresh catalyst, subsequently replacing the desulfurization catalyst in the latter stage with demetallization catalyst, replacing the demetallization catalyst in the former stage with desulfurization catalyst, and reversing the order of oil passage to prolong the service life of active catalyst (refer to U.S. Pat. No. 3,985,643); and the like processes.
Nevertheless the aforesaid process (6) involves the problem that the use of a plurality of preliminary reactors complicates the process equipment, thus increasing the equipment cost and running cost. The process (7) causes the problem that the replacement of the catalyst increases the catalysts costs and forces the discontinuance of operation during the course of the treatment. The actual situation at the present time is that a satisfactory process capable of readily inexpensively prolonging the service life of the catalyst has not yet been achieved.
Aside from the foregoing, environmental destruction on global scale is presently becoming a problem. In particular, NOx which is formed accompanying the combustion of a fossil fuel and turns to acid rain destroys forest, and besides particulate NOx exerts evil influence on human body when being inhaled. The exhaust gas from a diesel engine which is the source of generating and migrating NOx and particulate thereof, must be treated with a post-treatment unit and a catalyst. Since the sulfur in gas oil causes the catalyst to be poisoned and deteriorated therewith and limits the service life of the catalyst, it is necessary to reduce the content of sulfur in gas oil in order to stably maintain the sufficient treatment performance of the catalyst for a long period of time. The upper limit of sulfur content in gas oil is aimed at 0.05% by weight worldwide by the presently reinforced regulation of sulfur content. It is hoped, however, to further reduce sulfur content in future, and there is a possibility that the regulation be further strengthened to restrict the sulfur content finally as low as 0.03% by weight. On the other hand, even if the sulfur content reduction is realized, it is important to maintain the practical quality inherent in gas oil and therefore it is sought to actualize a low sulfur gas-oil maintaining the practical performance thereof.
Numerous techniques have already been developed for the hydrodesulfurization of a hydrocarbon oil. The sulfur content reduction can be achieved usually by raising the desulfurization temperature from the technical viewpoint, but it is well known that the gas oil thus obtained is rapidly deteriorated in hue. For the purpose of improving the hue thus deteriorated, there are proposed a two-stage hydrogenating treatment process (refer to Japanese Patent Application Laid-Open No. 78/670/1993), the use of a catalyst comprising a noble metal and the like. The aforesaid proposals, however, involve the problems that the treatment unit is complicated or the catalyst is unreasonably expensive. It is thought that the substances causing coloration and deteriorating the hue are exemplified by polycyclic aromatic compounds having at least three rings such as benzanthracene, perylene, benzofluoranthene and benzopyrene. These substances don't originally exist in crude oil, but is formed by desulfurization reaction at an elevated temperature. On the contrary, high temperature reaction is required for the purpose of realizing low sulfur content, and thus it is difficult to allow sulfur content reduction through the desulfurization reaction at a high temperature to be consistent with the prevention of forming the substances causing coloration.